Abstract: A data transmission system using a transmission code in which one data state corresponds to a zero transmission level, such as the alternate mark inversion (AMI) code, includes at each terminal adaptive signal processing circuits such as an echo canceller (402) and/or an adaptive equalizer (409). Each terminal includes a decision circuit (408) an output of which controls a switch (410) which is operative to allow updating of the adaption of the echo canceller (402) and adaptive equalizer (409) only when a data state corresponding to a zero transmission level is detected.